Field of the Invention
The present invention relates to a display apparatus using a plurality of liquid-crystal panels and, more particularly, to a display apparatus such as a projector which is capable of displaying brightly a document written with small characters by a large number of pixels and displaying a color gradation and a moving picture.
With the recent spread of notebook type personal computers, display apparatus such as a liquid-crystal projector which is capable of enlarging the image displayed on a screen of a personal computer and projecting the enlarged image for the purpose of presentation have come into wide use, and there is a demand for a further improvement of a picture quality (bright and clear picture, high resolution, enhancement in color reproducibility, etc.) and a reduction in size of such display apparatuses.
Various techniques have been developed to meet such a demand. For example, the opening of each pixel of liquid crystal panel is enhanced, the light quantity of a light source is increased, a liquid-crystal panel which does not use polarized light is utilized, a liquid-crystal panel having a high resolution is adopted, the characteristics of a color filter are improved, and a pixel pitch on a liquid-crystal panel is reduced.
A projection optical system in a projector is composed of a light source such as a halogen lamp, a liquid-crystal panel and an optical system such as a mirror and a lens. Projectors are divided into two types depending on the optical system, i.e., a single-panel projector using one liquid-crystal panel as shown in FIG. 15A and a three-panel projector using three liquid-crystal panels as shown in FIG. 15B.
In the single-panel projector shown in FIG. 15A, the reference numeral 101 represents a light source such as a halogen lamp, 102 a liquid-crystal panel on which a picture is displayed, 103 a condenser lens for collecting light from the light source 101, 104 a projection lens for enlarging the picture displayed on the liquid-crystal panel 102 and displaying the enlarged picture on a screen, and 105 a screen. In the single-panel projector, light is projected from the light source 101 onto the picture displayed on the liquid-crystal panel 102, and the transmitted light is projected on the screen 105. Such a single-panel projector is characterized in that since only one panel is used, the size of the apparatus is small and the price thereof is low. However, since one pixel region is divided into three regions, i.e., red, green and blue regions, and the light transmittance/cutoff in each region is controlled in accordance with R data, G data and B data, the light transmittance is low and the picture becomes dark.
In the three-panel projector shown in FIG. 15B, the reference numeral 111 represents a light source such as a halogen lamp, 112 a dichroic mirror for reflecting red light which is contained in the white light from the light source 111 and transmitting the other (green and blue) light, 113 a dichroic mirror for reflecting green light and transmitting the other (red and blue) light, 115 a dichroic mirror for reflecting blue light and transmitting the other (red and green) light, 116 to 117 mirrors, 118 to 120 liquid-crystal panels on which pictures are displayed, 121 a projection lens for enlarging picture (color picture) which is obtained by synthesizing the red, blue and green pictures transmitted from the respective liquid-crystal panels and displaying the enlarged picture on a screen, and 122 a screen.
The light transmittance/cutoff of each pixel of the liquid-crystal panel 118 is controlled in accordance with the B data of the RGB color pixel data, the light transmittance/cutoff of each pixel of the liquid-crystal panel 119 is controlled in accordance with the G data of the RGB color pixel data, and the light transmittance/cutoff of each pixel of the liquid-crystal panel 120 is controlled in accordance with the R data of the RGB color pixel data. As a result, the red, blue and green synthesized picture (color picture) is output from the dichroic mirror 115, and the picture is enlarged and displayed on the screen 122 by the projection lens 121.
According to such a three-panel projector, since light passes all the regions of one pixel, a brighter picture is obtained than a picture produced by single-panel projector. However, since three liquid-crystal panels are used, a large-scale structure is necessary and hence, the cost is unfavorably high.
The liquid-crystal panel has polarizing plates 132, 133 on both sides of a liquid-crystal layer 131, as shown in FIG. 16. The polarizing plate 132 transmits, for example, S-polarized light and the polarizing plate 133 transmits P-polarized light. The S-polarized light which passes the polarizing plate 132 rotates in the liquid-crystal layer 131, passes the transparent portion, goes out as P-polarized light and passes the polarizing plate 133.
Projectors are also divided into two types depending upon the projection form. One is a projector which projects a picture onto a screen on a wall, and the other is a projector provided with a screen such as a TV set which projects a picture from the back side of the screen.
As a liquid-crystal panel, a TFT (Thin Film Transistor) liquid-crystal panel is utilized which adopts a combination of a TN (Twisted Nematic) display mode and active matrix drive by the TFT. The TFT liquid-crystal is characterized by a high contrast, display of a color gradation, and a comparatively high speed (which enables display of a moving picture).
In the TFT liquid-crystal panel which is utilized in a conventional display apparatus such as a projector, an increase in the number of pixels leads to a rise in the cost, so that the maximum number of pixels is about 1280.times.1024 in the present state. It is therefore impossible for the TFT liquid-crystal to display an image of the size A4 (210.times.297 mm) in one screen. If it is obliged to display the image of the size A4 in one screen, the resolution is degenerated, so that the characters blur and become difficult to read, much less an image of a larger size such as B4 (257.times.364 mm) and A3 (297.times.420 mm). That is, it is impossible to display a document for such a large size in one screen of a TFT liquid-crystal panel with legible characters.
In addition, since the TFT liquid-crystal requires the two polarizing plates 132, 133, as shown in FIG. 16, as a property of the liquid-crystal material, it is only less than 1/2 (ordinary transmittance is 35 to 40%) of the total light quantity of the light source that can be utilized, so that the light transmittance is low (about 20% in monochrome display such as black-and-white display, about 5% in color display) and the picture becomes comparatively dark and indistinct.
A panel which dispenses with a polarizing plate has been proposed as a liquid-crystal panel which has a large number of pixels and which is bright. For example, it has been confirmed that nematic-cholesteric phase transition liquid-crystal utilizing the light transmittance and scattering dispenses with a polarizing plate and realizes as many as 3500.times.2500 pixels at its maximum (by Mochizuki et al, Super High Information Content Projection Display Using an NCPT Liquid Crystal, FSTJ, vol 128, 3, PP 369 to 376, 1992). Since there is no light absorption by a polarizing plate, the picture is bright; for example, the light transmittance is not less than 60% in monochrome display (black-and-white display). According to this liquid-crystal, it is possible to display an image of the size B4 or A3 with high definition and brightness to the same extent as in printing unlike the TFT liquid-crystal. However, since it takes several seconds to rewrite on this liquid-crystal panel and the rewriting speed is lower as compared with the rewriting speed by a CRT, which is several ten milliseconds, it is impossible to display a moving picture. In addition, it is impossible to display a color gradation.